This invention relates generally to throttle bushings in various types of pump systems and the like in which the throttle bushing surrounds at least a portion of a rotating shaft, and more particularly to a new and improved throttle bushing including improved means for preventing the entrance of foreign materials such as debris, contaminants, and other undesirable substances between the shaft and bushing and for expelling such materials which nonetheless enter therebetween.
Pump systems can be considered to straddle differential pressure zones or regions of relatively low and high pressure. Rotating shafts in the pumps are frequently employed to drive water or other fluids between these low and high pressure zones. In many cases, the low pressure zone is below atmospheric pressure. Driving or pumping water can for instance be accomplished by rotating impellers mounted on the shaft as shown in FIG. 1 below.
Since the shaft itself is exposed to each of these pressure zones (low and high), the water or fluid being pumped tends undesirably to leak or pass along the surface of the shaft. This tends not to conform to the primary or desired flow scheme for which the pumps are designed, even though it is known and somewhat inevitable that some leakage will be present in all systems. Seals are normally provided to control leakage flow of water or fluid along a shaft. However, seals generally are effective only for handling small pressure differentials for pressure differentials more than about 5 psi, more than only seals for preventing leakage are required. Otherwise, the seal itself will be damaged by the flow of water driven under high pressure. This causes the pump to become inoperable within an unreasonably short term or period of time, when exposed to excessively high pressure drops.
Accordingly, the use of bushings with seals rather than as an alternative to seals controlling high pressure leakage along the shaft surface has come into use. The higher the pressure differential traversed by the shaft, the more important is the bushing and its operational design or construction.
Aside from preventing leakage, the bushing can contribute to the lubrication of bearing surfaces for the rotating shaft. Additionally, the bushing can provide a structure through which cooling ducts can carry away heat generated during pumping operation, which might otherwise interfere with proper operation of the pump equipment according to its design requirements.
One notable problem encountered in pump systems employing rotating shafts arises from the presence of debris and solid contaminants in the fluid being pumped. Such contaminants can range from substances or materials such as dirt and sand to metal turnings and rust. These can lodge between the bushing and the shaft, damaging the metal surfaces on both the inner side of the bushing and the outer side on the shaft itself. This may additionally cause irregularities such as "galling." These irregularities permit unacceptable amounts of leakage along the bushing surface. Furthermore, debris or other materials lodged between the bushing and the shaft can cause the shaft to bind and lock, possibly causing complete failure of the pump system during operation.
To resolve such problems, grooves of various configurations such as a helical pattern have heretofore been formed in bushings to collect and expel undesirable material that enters the bushing. Additionally, in one instance, a circumferential end recess had been formed in a bushing, but this recess was larger than the connected helical groove. The larger recess allowed oversized debris to enter between the bushing and the shaft and lodge at the mouth of the helical groove. All of the prior grooved bushings share common problems with respect to oversized debris. The grooves cannot handle any debris larger than the cross-sectional areas of the grooves themselves and thus, when larger debris enter the pump system, it lodges between the inner bushing surface and the shaft surface and causes the above-mentioned irregularities and galling.
Another problem remaining in the field is the build-up of debris due to insufficient flushing action whereby the debris is not disposed of quickly enough.
An additional problem occurs when the bushing and the shaft are made of like materials. If the like materials are used, for instance type 410 stainless steel, the affinity of the metal for itself will cause a transfer of metal between the inner bushing surface and the shaft, raising metal burrs on the bushing and shaft, as well as creating more debris.
In light of the above problems, an object of this invention is to provide a bushing effective for deterring relatively large chunks of debris from ever entering between the bushing and the shaft.
Another object of the invention is to collect debris which enters at the higher pressure end of the bushing and to quickly pass the debris to the lower pressure end of the bushing for expelling it, in order to avoid damage to the inner bushing surface and the outer surface of the shaft.